The present invention relates to synchronous rectification control device in an insulated type switching power supply and a method of controlling the insulated type switching power supply in which switching elements in the primary side circuit are ON/OFF controlled and synchronous rectification is conducted in the secondary circuit to obtain a specified constant voltage DC output.
An insulated type switching power supply is a type of a commonly known converter in which an input (primary) side and an output (secondary) side are insulated by a transformer, switching elements provided in the primary side being ON/OFF operated to transfer energy to the secondary side through the transformer, and a rectifying circuit provided in the secondary side provides a constant voltage DC output
A structure called a full bridge converter is known for constructing an insulated type converter. A full bridge converter, shown in FIG. 8A for example, constructs a full bridge with four switching elements, transistors Tr1, Tr2, Tr3, and Tr4, which are generally MOSFETs (metal oxide semiconductor field effect transistors). In classical PWM control, transistors Tr1 and Tr4 are simultaneously turned ON generating a positive output and transistors Tr2 and Tr3 are simultaneously turned ON generating a negative output. An output voltage is controlled by varying a time percentage (a duty ratio), which is a percentage of ON period of each transistor Tr1, Tr2, Tr3 or Tr4 in a total time of ON period and OFF period. This control scheme may create a high impedance condition between both terminals of a load connected to the full bridge circuit, which is problematic in the case with a load including an inductance element. Accordingly, a phase shift scheme is often employed (see Non-patent Document 1, Patent Documents 2 and 3).
A phase shift full bridge converter, among various types of power converting schemes, is attracting attention for possibility of high output power, and nowadays employed extensively. A phase shift full bridge converter holds a time percentage of each of two half bridges, one half bridge having Tr1 and Tr2 and the other half bridge having Tr3 and Tr4, to be a fixed value of 50%. A specified output is obtained by adjusting a phase shift degree of ON/OFF operation of the two half bridges. The phase shift degree varies a time percentage of “a period in which a voltage equivalent to an input voltage is applied between both terminals of a load connected to the full bridge circuit” in a total period of this period and “a period in which voltages at both terminals are equal.”
FIG. 8A shows a structure of a known phase shift full bridge converter. A full bridge circuit in FIG. 8A comprises four switching elements that are MOSFET transistors Tr1 (101), Tr2 (102), Tr3 (103), and Tr4 (104) as described above. The four transistors Tr1 to Tr4 are operated such that the two half bridges having Tr1 and Tr2 and having Tr3 and Tr4 are held at a time percentage of fixed value of 50%, and a phase of ON/OFF operation of the two half bridges is shifted by supplying a gate of each transistor with a control signal from a phase shift control IC (150) through a pulse transformer 111 or 112. A pulse voltage is given to a primary winding of a transformer 120 from a node 113 of a source terminal of the transistor Tr1 (101) and a drain terminal of the Tr2 (102) through an inductor Lz (105) for soft switching; and another pulse voltage is given to the primary windings of the transformer 120 from a node 114 of a source terminal of the transistor Tr3 (103) and a drain terminal of the transistor Tr4 (104) through a capacitor 106. The control system finally transfers energy to a smoothing reactor 131 and a smoothing capacitor 132 provided in the secondary side through a secondary windings of the transformer 120. The whole of the secondary windings of the transformer 120 is divided into two opposing sections of windings. The two sections of windings are connected to a rectifying diode 141 and a rectifying diode 142 from two distinct terminals of the two sections of windings. Since a commercially available phase shift control IC (150) is used, details thereof are not described here.
The phase shift full bridge converter of FIG. 8A gives a specified constant voltage DC output by rectifying the output of the secondary windings using the rectifying diodes 141 and 142. FIG. 8B on the other hand shows a phase shift full bridge converter to which a synchronous rectifying scheme is applied. The converter of FIG. 8B uses switching elements 161 and 162 of MOSFET transistors, for example, in place of the rectifying diodes 141 and 142. A synchronous rectification control circuit 160 controls to turn ON the switching elements 161 and 162, according to a signal from a timing detection means (not shown in the figure) provided in the primary side circuit, at timings of the conventional electrical continuity of diodes. A synchronous rectification system of such a construction is intended to improve power conversion efficiency.
In a converter employing the synchronous rectification system of FIG. 8B for improving power conversion efficiency, it is important to control a forward current in the diodes connected in parallel to the MOSFETs of switching elements 161 and 162 to flow in the main switching elements of the MOSFETs as much as possible by means of a synchronous rectification control circuit 160. Besides, the diodes accompanying the switching elements 161 and 162 must block reversed current. Therefore, it is critical to control timings of turning ON/OFF the MOSFETs of synchronous rectifying transistors by the synchronous rectification control circuit 160.
Some systems employing synchronous rectification scheme in the secondary side are known as described below. FIG. 9 shows an example (referred to as ‘a conventional system (1)’ in the following description) in which voltages of the opposing sections of windings in the secondary side obtained through resistors 171 and 172 are directly utilized for gate signals to synchronous rectification switches 161 and 162 in the secondary side. An example employing the conventional system (1) is disclosed as FIG. 1 in Patent Document 1. In FIG. 1 of Patent Document 1, synchronous rectification transistors Q1 and Q2, which are MOSFETs having inverse parallel diodes, are connected to secondary windings of a transformer XFRMR. The secondary windings drive an LC circuit composing an output section. The synchronous rectification transistors Q1 and Q2 in this construction are connected in a configuration of so-called cross-connected switches, which means that gate electrodes are connected to opposite terminals of the secondary windings of the transformer XFRMR.
FIG. 10 shows another example (referred to as ‘a conventional system (2)’ in the following description) employing a synchronous rectification scheme in the secondary side. A phase shift control IC (150) in the conventional system (2) transfers timing signals as same as those for a primary side switching control directly to gate terminals of secondary side synchronous rectification switches 161 and 162 through drivers 181 and 182.    Patent Document 1    Japanese Patent No. 409-4727 (FIG. 1 in particular)    Patent Document 2    Japanese Unexamined Patent Application Publication No. 2005-348567    Patent Document 3    Japanese Unexamined Patent Application Publication No. 2008-113473    Non-patent Document 1    Tamotsu Inaba: “Manufacturing a phase shift PWM scheme ZVS variable power supply” (in Japanese) in a journal “Transistor Technology” published by CQ Publishing Co., Ltd., Vol. 41, No. 6, pages 228-236, Jun. 1, 2004.
The conventional full bridge converters employing a secondary side synchronous rectification scheme as shown in FIG. 9 and FIG. 10 have the following problems when a phase shift scheme is applied to the converters. FIG. 11A shows operation waveforms and synchronous rectification capability range of a phase shift full bridge converter of the conventional system (1) as shown in FIG. 9 employing a secondary side synchronous rectification scheme. The waveforms indicated by the symbols VNP, VDS2, VDS1, and INS1 are voltage and current waveforms corresponding to the voltages and a current indicated by the same symbols in FIG. 8B. FIG. 11B shows operation waveforms and synchronous rectification capability range of a phase shift full bridge converter of the conventional system (2) as shown in FIG. 10 employing a secondary side synchronous rectification scheme. The waveforms indicated by the symbols VNP, VDS2. VDS1 and INS1 are voltage and current waveforms corresponding to the voltages and a current indicated by the same symbols in FIG. 8B like for the case of FIG. 11A. FIG. 11A and FIG. 11B show an operation only in one of the two sections of the secondary windings. An operation in the other section of the windings is similar to the operation in FIG. 11A and FIG. 11B, so the illustration is omitted.
The conventional system (1) shown in FIG. 9 employing a secondary side synchronous rectification scheme cannot supply current in the MOSFETs of the secondary side synchronous rectifying switches in a substantial period out of the total current-flowing period as shown in FIG. 11A resulting in generation of rather large losses. This is a problem that actual capability range of synchronous rectification is substantially narrow in comparison with an ideal synchronous rectification capability range.
The conventional system (2) shown in FIG. 10 employing the secondary side synchronous rectification scheme cannot manage the remaining current (INS1 flowing in the period td indicated in FIG. 11B) flowing due to affection of the inductance of the inductor Lz (105) for ZVS (zero voltage switching) and the parasitic inductance (not illustrated) to flow in the secondary side synchronous rectification switch 161. This situation too, generates rather large losses, and has a problem that the actual capability range of synchronous rectification is substantially narrow in comparison with an ideal synchronous rectification capability range, like in the conventional system (1). In operation of the conventional system (2), the current INS1 and INS2 does not flow in completely exclusive manner; ‘completely exclusive’ means that when a current is flowing in one of the two circuits, the other circuit carries zero current. At a moment of reversing a voltage VNP applied to the primary windings, that is, a moment the voltage VNP that has been null until this moment starts to rise up or fall down, a current in one section of the secondary windings that has been mainly carrying whole current does not abruptly vanishes, but attenuates rapidly in a period td. The other section of the secondary windings carries a current rapidly increasing corresponding to the attenuating current. Thus, the current corresponding to the attenuating current cannot be carried by the secondary side synchronous rectifying switch in the period td during which the current carried by the section of the secondary windings that has been conducting substantially whole current.
In addition, means for signal transfer from a primary side circuit to a secondary side circuit (for example, the drivers 181 and 182) is necessary. Moreover, the transfer means needs to be an insulated type in an insulated type power supply. Thus, there exist problems in a number of parts, a layout area, and a cost increase.
It is therefore an object of the present invention to provide a synchronous rectification control device, a control method, and an insulated type switching power supply capable of synchronous rectification control with high power conversion efficiency without supplying additional signal to the secondary side from the primary side.
Further objects and advantages of the invention will be apparent from the following description of the invention.